Metallic copper is an excellent material for chip interconnection due to its advantages such as good electrical conductivity, high thermal conductivity, low melting point and extensibility. Copper plating is a method of choice for chip interconnection. However, with continued minimization of the chip line width, it becomes harder and harder to deposit copper metal lines without defects. A defect-free copper plating is achieved by integrating seamlessly the plating chemistry, plating tool, and semiconductor chips.
The challenges in acid copper plating have always been: 1) leveling effect, 2) said leveling effect over a wide range of aspect ratio (depth:width) preferably found on the same die, 3) achieving the desirable leveling effect at high current densities therefore increasing the manufacturing throughput, 4) levelers that are non-toxic, environmental friendly, 5) levelers that are easily analyzable by conventional means so their concentrations can be monitored and controlled in a plating bath, and 6) minimizing by-products which often have detrimental effect on bath stability. The quality of the leveler in an acid copper plating chemistry determines the quality of copper pillars, under bump metallurgy (UBM), redistribution lines (RDLs) as well as its ability to fill through silicon vias (TSVs). Most of the MEMs, LED, and semiconductor customers require flat and smooth surfaces but there are some applications that require convex surfaces. The selection and optimization of electroplating conditions, especially the plating chemistry, play a key role in obtaining desired surface topographies. Many of the users of such plating chemistries are large semiconductor manufacturers integrated device manufacturers (IDMs), or packaging houses who typically process semiconductor chips with different geometries, dimensions, including different heights. The manufacturers and packaging houses are making these semiconductor chips for fabless companies, IDMs and/or end users. Because the design of each and every one of these companies is different, this requires that the plating processes employed by the manufacturers and packaging houses are versatile and have a wide process window. For example, a foundry makes semiconductor chips for fabless companies and IDMs. One of the steps is to electroplate copper pillars with via diameters ranging from about 10 μm to about 200 μm, and the height ranging from about 20 μm to about 150 μm. If the fab could use a single copper plating chemistry to meet all of its customer requirements on feature topography and within die uniformity, it would significantly reduce its manufacturing cost. If the foundry uses multiple copper plating chemistries to cover the full range of the feature dimension, it would increase the manufacturing cost because it has to deal with not only a number of different chemistries, but also associated cost for product inventory, process control and maintenance, etc. Currently, in the market, there is no single commercial copper plating chemistry that could produce microchips with a wide range of feature dimensions at high yield, nor could a single commercial copper plating chemistry produce flat or convex topography by simply adjusting the concentration or composition. In addition, not a single commercial copper plating chemistry could produce same topography for features from about 10 μm to about 150 μm at deposition rate as high as 10 A/dm2(ASD) or 2.2 μm/minute. A typical acid copper plating chemistry includes virgin makeup solution (VMS), which includes a metal salt, an acid, chloride ions and organic additives. The content of organic additives is typically very low (at ppm level), but they determine the surface features as well as bulk properties of the electroplated layer. Organic additives can be categorized as a suppressor (or wetting agent), a leveler (or grain refiner), and an accelerator (or brightener). A suppressor acts as a wetting agent which helps to wet the metal surface so plating can take place. During deposition, it suppresses the growth rate of the deposited metal so it can grow through a layer by layer mechanism. Consequently, it results in adhesive, smooth metal surfaces without dendrites. A copper plating bath containing only a suppressor produces a matte or dull surface. To this composition, one can add a leveler. By definition, it levels or fills the “potholes” of the deposited surface in such a way that the height difference between the highest point and the lowest point of a given feature is minimized. In addition, a good leveler also ensures the height difference between the tallest and the shortest bump in a die is minimized. A copper plating bath that contains both a suppressor and a leveler produces a surface that is somewhat reflective but not bright. To this composition, one can add an accelerator. By definition, it increases the copper deposition rate at the deposition potential. At the same time, it results in a reflective, shining surface. In semiconductor copper plating, the most critical component is the leveler, because ultimately it determines the within die uniformity, which largely determines the yield. Although choosing the right suppressor is also crucial, especially in the case of copper damascene, the core technology innovation today centers around the leveler for UBM, RDL, copper pillar and TSV plating. This is because taller features and larger dimensions are required for these applications (height 3 μm to 200 μm, line width 2 μm to 20 μm, diameter 10 to 200 μm), therefore maintaining yield at >99.9% becomes extremely challenging. In addition, when taller copper pillar is required, it is critical to deposit copper at high deposition rate such as 10 ASD (2.2 μm/min) or above so high productivity can be achieved. This allows the manufacturer to reduce the unit cost therefore to maintain its competitiveness. Clearly there is a need for a versatile electrolytic copper deposition chemistry. As described below, the present invention offers such a solution by employing quaternary ammonium salts of dialkylaminoalkyl esters of 10-thiaxanthenecarboxylic acid and its derivatives as levelers. Although there are commercial copper plating baths that could meet one or more of the six challenges mentioned previously, there is none that could meet all six. The composition and method of present invention could be as described below.